The goal of this competing continuation is to continue studies of mechanicsof the heart. We have been interested in the dynamics of contraction, the long term goal being to relate properties at the sarcomere level to contraction in the myocardium. The present experiments will be carried out principally on ventricular trabeculae. We plan to measure properties such as force-velocity relations, extent of shortening, and effects of variations in contractility, all at the sarcomere level. Several methods, some new, will be used to measure sarcomere dynamics with high resolution. We will attempt to determine the role of sarcomere shortening steps in contractile dynamics. This information will be of use in elaborating contractile mechanisms, and in establishing basic principles of cardiac dynamics.